VSAT systems consist of three major components: a hub, the satellite, and the VSATs themselves. These are usually configured as a star topology network. Messages from the VSATs are sent over a channel shared either by time division multiple access (TDMA) or frequency division multiple access (FDMA). These access methods allow each VSAT to have all of the available bandwidth part of the time or part of the bandwidth all of the time. TDMA schemes are more responsive to growing and changing networks. Messages moving from the hub to the VSATs are transmitted over a single time division multiplexed (TDM) channel. The distinction between TDMA and TDM is that in the former, many stations are transmitting on a shared channel while in the latter only one station is transmitting. VSATs listen to the entire TDM data stream but “grab” only those packets addressed to it.
First, a hub with a large (4-8 m diameter) antenna and front end processor acts as the central network control point and as the main data processor. It is generally located at or near the central offices or corporate headquarters. Network management functions such as protocol and frame changes, frequency and time slot assignment. Each hub is capable of handling several thousand VSATs. If a network is not very large, then this huge capacity is not needed and a VSAT network with a dedicated hub may not be economical due to the large initial investment necessary to establish the hub. In order to make VSAT networks a viable alternative for smaller networks, many VSAT vendors have established regional hubs that are shared by several corporate networks. These hubs are owned and operated by the VSAT supplier and the capabilities are “leased” by the users, alleviating the need for both the large capital investment and the cost of a staff dedicated to running the hub.
The second component is a communications satellite. These satellites are in a geostationary orbit approximately 42,200 km above the equator. From this position, a global coverage beam is able to the Earth’s surface. In order to provide high gain for the coverage areas, multiple beams may be used. The VSAT user leases a transponder in the appropriate coverage region, or a portion of the transponder’s bandwidth, from the VSAT provider. Station keeping and health and welfare of all systems aboard the satellite are the responsibility of the satellite’s owner.
To the users of VSATs, the satellite is a relay that takes in all signals on the uplink frequency, shifts them to the downlink frequency, and then retransmits them. The most notable characteristic of the satellite link is the added propagation delay. As a rule of thumb, a signal will take 0.27 seconds to propagate from the sender to the receiver along a single hop satellite link. If there are two hops between sender and receiver, such as might be the case in a star network, then the delay is 0.54 seconds. While this delay may not be intolerable, it would certainly be an annoyance to the user.
Finally, the VSAT itself is a small (.8- 4.5 m) antenna, an outdoor unit consisting of a transmitter (1-20 Watt) and receiver, and an indoor unit with a modem, encoder/decoder, multiplexer/demultiplexer, and digital data interface. The modular design allows easy upgrades of system components and ensures transportability. Because of the small antenna and low transmitter power, communications are normally limited to VSAT-to-hub or hub-to-VSAT, though systems are available that allow VSAT-to-VSAT messages. Data rates for low-end VSATs typically range from 56 to 128 kbps.
Frequency bands used in satellite communications are generally referred to by a letter designation. This is a holdover from World War II attempts to hide exact radar frequencies from the enemy. Through the years the letter designations were declassified, modified, and generally abused. It is unknown whether this subterfuge confused our opponents, but it certainly causes difficulties for communication engineers since there is no recognized standard. The two frequency bands of interest to VSAT communication are the C and Ku bands. Generally, the C band of the radio frequency spectrum is considered to range from 3.7 to 6.425 GHz while the Ku band extends from 10.7 to 18 GHz.
The first VSAT networks used C-band satellites for one way point-to-multipoint communications. These early VSATs had antennas of 0.6 to 1.2 m and spread spectrum technology in order to minimize interference with terrestrial microwave and adjacent satellites. The need for an interactive capability led to the introduction of two-way VSATs using C-band, but these systems were limited to bit rates of around 9600 bps and suffered from interference problems. Today, C-band VSATs are used by news organizations and others that are primarily concerned with broadcasting information to geographically dispersed locations. Most new VSAT data networks utilize Ku-band satellite channels. These are free from ground-based microwave interference and offer a larger available bandwidth, hence higher potential data rate. These networks have found a wide variety of uses in corporate data communications:
- Point-of-sale information is gathered and transmitted to central computers for order processing, credit authorization, and inventory control.
- Automatic teller machines are connected to the central office for transaction approval and processing.
- Terminals are connected to a central database for use in hotel and airline reservation systems.
- Corporate teleconferencing and private phone systems.
- Broadcast of corporate training films and in-store audio/video advertisements to branches.
ADVANTAGES OF VSAT SYSTEMS
Service on a VSAT network can be entirely controlled by the user. Installation and testing of the new VSAT can be done by personnel in the using organization. In order to integrate a new VSAT into a network, network management must make frame changes that allow the new VSAT to access the link. This is done through commands issued from the network hub, which is run by the using organization. There is no outside agency that must be dealt with in case there are changes that must be made to the network.
If there are any difficulties with the service, the user is dependent upon the leased line provider for tracking down and correcting the problem. Since the 30 deregulations of the telephone system, this is increasingly difficult. The local phone
system may blame the regional carrier, who can blame the next region, and so on until the other end of the line is reached. Any disputes over service and billing must be negotiated to the satisfaction of all parties. When difficulties crop up with a VSAT network there are no disputes over who is at fault. The blame rests squarely with the VSAT provider, allowing a rapid resolution.
Natural disasters such as earthquakes and unnatural ones such as a backhoe slicing through a cable can wipe out communications paths on a terrestrial network. VSAT networks are immune to this type of problem. Also, VSATs are capable of providing a lower bit error rate than that of leased data lines.
Modifying a terrestrial network can be both costly and time-consuming In order to add nodes, it is necessary to install leased lines to the site. The user has no direct control over the installation an setup process because he is dependent upon an outside agency for service. It is entirely possible that delays of weeks or months may be encountered before service can be initiated. Installing or moving nodes on a VSAT network is simple. All components are modular and easily transported to the site. The installation procedure is simply erecting and pointing the antenna and attaching the interface to the local network. Any necessary frame changes can be initiated from the hub.
Leased line pricing usually uses a 1.544 Mbps line, or Tl carrier, as a basis for cost comparison. Price for a line increases as a function of distance and required bit rate. A typical 1000 mile long Tl link costs about $10,000 per month while a 500 mile Tl link runs about $6,000. If a full 1.544 Mbps is not needed, a fractional Tl can be leased 31 at rates of 384, 512, and 768 kbps. A 1,000 mile long 512 kbps line will cost approximately $7,000 per month. These prices do not include the cost of any terminating equipment and circuits needed ar either end of the line.
Prices on VSAT remote sites range from $6,000 to $20,000. For small networks of less than 200 remote sites, shared hubs allow many networks to utilize the same central ground station for network control. These shared hubs are owned and operated by the VSAT provider, thus the initial capital outlay costs. For larger networks, it becomes economical to operate a dedicated hub for networks of between 200-300 nodes.